The preparation of aromatic polycarbonates by the melt transesterification process is known and is described, for example, in “Schnell”, Chemistry and Physics of Polycarbonats, Polymer Reviews, Vol. 9, Interscience Publishers, New York, London, Sydney 1964, in D. C. Prevorsek, B. T. Debona and Y. Kersten, Corporate Research Center, Allied Chemical Corporation, Moristown, N.J. 07960, “Synthesis of Poly(ester)carbonate Copolymers” in Journal of Polymer Science, Polymer Chemistry Edition, Vol. 19, 75-90 (1980), in D. Freitag, U. Grigo, P. R. Müller, N. Nouvertne, BAYER AG, “Polycarbonates” in Encyclopedia of Polymere Science and Engineering, Vol. 11, Second Edition, 1988, pages 648-718 and finally in Dr. U. Grigo, K. Kircher and P. R. Müller “Polycarbonate” in Becker/Braun, Kunststoff-Handbuch [Plastics Handbook], Volume 3/1, Polycarbonate, Polyacetale, Polyester, Celluloseester [Polycarbonates, polyacetals, polyesters, cellulose esters], Carl Hanser Verlag, Munich, Vienna 1992, pages 117-299.
Said process was developed mainly for the transesterification of diphenyl carbonate with bisphenol-A. The transesterification of ester-substituted diaryl carbonates, such as bis-methyl salicyl carbonate, with bisphenol A is also known, e.g. from US 2005/0261460 A1.
Due to their high heat resistance, polycarbonates are used, inter alia, in fields in which a relatively high level of thermal stress is likely to occur. Specific (co)polycarbonates based on cycloalkylidendiphenols, an example being a copolycarbonate based on bisphenol A and bisphenol TMC (1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane) are known to be particularly heat resistant. The production and use of such polycarbonates based on cycloalkylidenediphenols is described, for example, in DE 3 903 103 A1, EP 414 083 A2 and EP 359 953 A1.
During the production of specialty polycarbonates with high temperature resistance by means of the known transesterification process, the high temperature resistance is a drawback. Because of this difficulty, highly heat resistant polycarbonates are normally made by means of the interfacial polycondensation process with phosgene.
For example, highly heat resistant polycarbonates containing structural units derived from 1,1-Bis(4-hydroxyphenyl)-3,3,5-trimethyl-cyclohexane (bisphenol TMC) are not commercially prepared by the melt transesterification process. This is because due to the high temperatures required to form a melt, a large portion of diphenyl carbonate is lost into the vacuum system in the early stages of the reaction process and thus hampers phenol recovery. In addition, the early loss of diphenyl carbonate adversely affects the further polycondensation process, because it results in a highly viscous reaction mixture, which causes processing and reactivity issues leading to a high residual monomer content and low molecular weight of the final product as well as bad process reliability.
For this reason, complex work-around processes have been developed in the past for the preparation of highly heat resistant polycarbonates by means of the melt transesterification process (cf. DE4315035A1), and up to now, the commercial production of such polycarbonates is restricted to the interfacial technology.